This invention relates generally to improvements in a cascade device for a gas turbine, and more particularly to an improved cascade device having air cooling chambers provided respectively within leading and trailing edge portions of each stationary blade.
Recently, in order to enhance the performance of a gas turbine, the temperature of combustion gas has been raised more and more, so that stationary blades and moving blades of the gas turbine operate in a very thermally severe environment.
Therefore, these blades must be cooled by some cooling means.
Generally, for cooling turbine blades, there has been extensively used a method in which part of the compressed air for combustion purposes is taken out, and is caused to flow through a cavity (air cooling chamber) within the blade to cool the blade.
A typical example of such a cooling method is disclosed, for example, in Japanese Patent Unexamined Publication No. 2-241902, in which a cooling chamber (or flow passage) is provided within a trailing edge portion of a blade, and projections or pin-like members are provided within this cooling chamber so as to achieve a good heat exchange. With respect to cooling air serving as a cooling medium, the cooling air which has cooled the cooling chamber in the central portion or the leading edge portion of the stationary blade is led to the trailing edge portion of the stationary blade, or the cooling air is led directly to the cooling chamber in the trailing edge portion of the stationary blade, thereby cooling this trailing edge portion, and then the air raised to a high temperature is discharged from the trailing edge portion, thus cooling the blade.
On the other hand, the air taken out from the compressed air for combustion purposes is further used as sealing air. More specifically, a gap or clearance is formed between the stationary blade and the moving blade in order to prevent overshoot. Because of the provision of this gap at this portion, the leakage of the high-temperature gas naturally develops there, and therefore it is necessary to seal this portion. The air taken out from the air for combustion purposes is used to seal this portion. Generally, this sealing air is fed from an outlet of a compressor into a rotor, and fills in the gap at the cascade portion with a certain pressure.
In the cascade device of this construction, the trailing edge portion of the stationary blade is cooled by the air which has been supplied into the stationary blade to cool the inner wall of the stationary blade through impingement cooling or convection cooling; therefore, the temperature of the air is raised, and the pressure of the air is decreased. This results in a tendency that the trailing edge portion of the stationary blade fails to be sufficiently cooled. In the other method in which the cooling air is led directly to the trailing edge portion of the blade, the trailing edge portion can be cooled to a certain degree; however, since the air raised to a high temperature as a result of the cooling is discharged from the trailing edge of the blade to a main-stream operating gas passage, the velocity of the cooling air flowing through the cooling chamber is determined by the pressure difference between the pressure within the cooling chamber and the outlet pressure (i.e., the pressure at the outlet of the trailing edge portion of the blade), and therefore the velocity of the cooling air tends to become uneven. Namely, in the turbine blade, the pressure distribution in the radial direction of the main flow (stream) passage is not uniform because of a centrifugal action caused by the flow of the high-temperature operating gas. Namely, the pressure of the blade surface is high at the outer peripheral portion, and is low at the inner peripheral portion. Therefore, with the type of cooling construction in which the air is blown from the cooling flow passage to the trailing edge portion of the blade, the velocity of the cooling air flowing through the cooling chamber is uneven.
Incidentally, it is well known that the cooling characteristics of the cooling air become good in proportion to the velocity. Namely, the unevenness of the velocity causes the unevenness of the cooling characteristics, which results in a disadvantage that the temperature of the blade becomes high at the outer peripheral portion, and is low at the inner peripheral portion, so that a temperature difference develops at the surface and the inside of the blade in the direction of the height of the blade. Furthermore, when blowing the cooling air from the trailing edge portion of the blade, the high-temperature operating gas of high velocity is mixed with the low-temperature cooling air of low velocity, so that a so-called mixture loss develops. This results in a problem that the aerodynamic performance is degraded.
A more serious problem is that a large amount of compressed air is used as this cooling air and the sealing air, so that the air for the combustion is not secured in a sufficient amount, which results in a lower output of the gas turbine.